As jobs are picked up in the same order by background tasks as they are
when FinishAll is called it may happen that the main thread just waits
for background threads without doing any work. So first run jobs, that
are not running in background, to completion and then wait for remaining
jobs to finish.

BUG=v8:6093

Change-Id: Ica83db2a504771b633cfdfc4e95e1ac8e43111d6
Reviewed-on: https://chromium-review.googlesource.com/488244Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
Cr-Commit-Position: refs/heads/master@{#45017}